The invention relates generally to echo receiving systems and more particularly to a raster scan radar having a video memory for storing data in rectangular coordinates relative to a true origin.
As is well known in the art, marine pulse radars are used to detect the presence of objects which produce echo signals from transmitted pulses. Generally, an antenna scans in azimuth while transmitting pulses. When a transmitted pulse strikes an object such as, for example, a distant ship or land, returns are received back at the antenna in time as a function of the range of the objects from which they are reflected. Accordingly, the input to the radar receiver for one transmitted pulse can be characterized as a train of echos or pulses from objects at different distances. After demodulation to remove the radar's carrier fequency, this pulse train is known as radar video. The video from each transmitted pulse is identified as data in a plurality of successive range cells or bins. The video data is therefore initially defined by polar coordinates relative to own-ship which means that the data in each range cell is identified by an azimuth angle and a range value from the position of own-ship.
As is well known, it is desirable to visually display the radar returns for operator interpretation. One common radar display has been the plan position indicator (PPI) wherein the video data for successive transmitted pulses are displayed in successive sweeps from the center of the polar coordinate system on a CRT with a long persistence phosphor. A serious drawback of a PPI-type display is that it is refreshed only once per revolution of the radar antenna resulting in a relatively low intensity display that is difficult to see in daylight. Because of this and other drawbacks, more recent radar systems have commonly used a rasterscan display because its refresh rate is much higher and therefore provides an intensity which can be viewed in daylight.
Because it is necessary to present horizontal lines of data defined by rectangular or x,y coordinates to the CRT of a raster scan system, conversion processes have been used to convert the data which is received in polar coordinates relative to own-ship into cartesian coordinates. In the prior art, scan converters have been used to transfer data from range-azimuth memories into memories called bit image memories having one-to-one correspondence between the memory addresses and the x,y pixels or elemental display positions on a CRT. One type of prior art scan converter generates x,y and R addresses along the line defined by the azimuth angle of the present sweep so as to address x,y memory locations in a bit image memory (BIM) and, synchronously, address R memory locations in a memory containing the video range data for the present sweep.
In prior art scan converters, the x,y addresses for addressing the BIM have been generated relative to own-ship. In other words, even though the ship is moving, own-ship has corresponded to the same memory location which conventionally has been the center of the memory field. To display the data with relative motion which means own-ship is stationary at one location on the CRT, data is transferred line by line to the CRT from the corresponding region of the BIM surrounding own-ship. To display the data with true motion which means the map remains stationary and own-ship moves with respect to it, the portion of the BIM which is transferred to the CRT is continuously changed or modified.
There are serious drawbacks to the above-described prior art apparatus and method for storing data in a raster scan memory in accordance with cartesian coordinates relative to own-ship. First, because the coordinate origin is moving, stationary targets are stored at different storage locations in the memory from scan to scan. Accordingly, to provide scan-to-scan integration of the data actually stored in the memory, very complex and time consuming computations would be necessary to determine the storage location with which the present video data should be correlated. Second, because the CRT is refreshed at a much faster rate than the scan of new video data, there are CRT display regions having spatial discontinuities. Third, the memory field must be relatively large as compared to the display field so that the data can be displayed in ture motion no matter where own-ship is located in the display field.